Generally, when a watercraft is moored at a berthage, a hull of a watercraft may be subjected to physical damage by wave activity resulting from the wakes of passing watercraft.
Further, when the watercraft is sailed or submerged in water, particularly seawater for long period, about 600 types of marine life including shellfish such as barnacles, mussels and oysters are attached to the submerged hull of the watercraft. Such marine life attached to the hull of the watercraft lower the sailing speed of the watercraft on account of friction between the hull of the watercraft and the water. This may cause a significant problem because more than two times of cost of fuel may be required. In order to protect the hull of the watercraft from the attachment of such marine life the submerged hull of the watercraft was conventionally coated with antifouling paint. However, because most antifouling paint is toxic and causes seawater pollution, its use on the hull of the watercraft is regulated by International Maritime Organization (IMO). Low toxic antifouling paint has been developed in order to address this toxic problem. However, even if the low toxic antifouling paint is coated onto the hull of the watercraft, it may have a short effective life and may bring many problems in connection with the painting work.
The marine life attached to the surface of the hull should be physically removed on a cycle of about two to six months and the removal work entails great expense.
In order to solve or relieve the problems, the watercraft should be raised from the water so that new marine life may not attach to the hull and so that old marine life already attached to the hull may be necrotized, while the watercraft is not sailed and submerged in water for long time. There are many conventional lifting apparatuses to raise the watercraft from the water. However, because most conventional lifting apparatuses are built in platform or under water, they are prone to corrosion. Moreover, most conventional lifting apparatuses cannot adapt to the ebb and flow of the tide.